1. Field of the Invention
This invention relates in general to communication devices and in particular to communication devices incorporating a transmit signal path correction system with self-calibration capabilities.
2. Description of the Related Art
Non-idealities, such as direct current (DC) offsets and in-phase (I) and quadrature (Q) gain and phase imbalances, in the transmit signal path of a communication device can affect the quality of the modulated transmit signal. The specified non-idealities occur primarily due to circuit level mismatches in the analog quadrature circuitry at baseband as well as in the radio frequency (RF) and/or intermediate frequency (IF) signal paths. These transmit path non-idealities lead to degradation in the error vector magnitude (EVM), uplink bit error rate (BER), and the required transmit spectral mask performance. Also, in the case of 3G Wideband Code Division Multiple Access (WCDMA) transceivers, such non-idealities lead to degradation in Peak Code Domain Error (PCDE) performance.
To be able to meet performance requirements, the offsets and imbalances due to the baseband quadrature circuits must achieve difficult performance goals to allow for sufficient headroom for non-idealities in the I/Q modulator, RF/IF gain control, and power amplifier stages.
FIG. 1 illustrates a block diagram of a conventional I/Q modulated transmit signal path 10 within a communication device. An I-channel digital pulse-shaping filter 12 digitally filters an I-channel signal 14 and also provides for spectrum control. A Q-channel digital pulse-shaping filter 16 digitally filters a Q-channel signal 18 and also provides for spectrum control. Coupled to the I-channel digital pulse shaping filter 12 is an I-channel digital to analog (D/A) converter 20, receiving the output of the I-channel digital pulse-shaping filter 12, which converts the digital signal received from the I-channel digital pulse-shaping filter 12 into analog format while supporting sufficient dynamic range to meet the transmit performance requirements for a given communications protocol. Similarly, coupled to the Q-channel digital pulse shaping filter 16 is a Q-channel digital to analog (D/A) converter 22 receiving the output of the Q-channel digital pulse-shaping filter 16, which converts the digital signal received from the Q-channel digital pulse-shaping filter 16 into analog format while supporting sufficient dynamic range to meet the transmit performance requirements for a given communications protocol. Coupled to the I-channel digital to analog (D/A) converter 20 is an I-channel analog reconstruction filter 24 receiving the output of the I-channel digital to analog (D/A) converter 20, and providing means to eliminate the aliasing signal bands and to limit the far out quantization noise. Similarly, coupled to the Q-channel digital to analog (D/A) converter 22 is a Q-channel analog reconstruction filter 26 receiving the output of the Q-channel digital to analog (D/A) converter 22, and providing means to eliminate the aliasing signal bands and to limit the far out quantization noise. An I-channel analog attenuation and gain stage 28 is coupled to the output of the I-channel reconstruction filter 24 to support signal levels required at the input/output (I/O) of chip boundaries and also to support baseband power control schemes. Similarly, a Q-channel analog attenuation and gain stage 30 is coupled to the output of the Q-channel reconstruction filter 26 to support signal levels required at the input/output (I/O) of chip boundaries and also to support baseband power control schemes. Coupled to the outputs of the I-channel analog attenuation and gain stage 28 and the Q-channel analog attenuation and gain stage 30 is an I/Q modulator 32 to create the complex signal prior to gain control using voltage controlled amplifier (VCA) stages and a power amplifier (PA).
Sources of baseband DC offsets and I/Q imbalances include the I-channel D/A converter 20 and the Q-channel D/A converter 22, the I-channel reconstruction filter 24 and the Q-channel reconstruction filter 26, the I channel analog attenuation and gain stage 28 and the Q-channel analog attenuation and gain stage 30. Such offsets and imbalances can vary over process, temperature, and supply voltage; thus, they should be eliminated in a high performance but systematic manner to meet the stringent modulation accuracy requirements of today's communication systems.
Therefore, what is needed is a high performance, low cost and low power system for correction of non-idealities such as direct current (DC) offsets and I/Q quadrature gain and phase imbalances in the transmit signal path of a communication device.